Trailer cruiser revisited... as a trimaran

[Ad Hoc]

Matt

So long as you are happy with the end result. Our advice is just that, since at the end of the day, it is your boat and have to live with it and all its compromises.

Just because our advice is "technically" the better solution, doesn't always make it the most practical for your application!

PS..don't forget to round the chine from about 1/3rd aft length to the transom

[marshmat]

My latest tweaking and attempts to keep the bottom panels developable have led me to conclude that just switching to strip plank for the main hull underside would allow a shape I'm happier with, at no real penalty in total build time. It's essentially the same main parameters and curve of areas as before, just smoothed out a bit.

I know none of us trust CFD (myself included)- but until the Warm Season begins in another seven months, it's all I have. The calculations for the shape shown in the attached screenshots look promising. (Total drag on the order of 2.9 kN at 20 knots, I estimate an uncertainty of about +- 0.3 kN in that figure).

Fully loaded (~2500 kg):

          Vaka     Ama
LWL      8.00     6.73    m
BWL      0.86     0.41    m
T        0.46     0.26    m
Disp     1920     290     kg
L/B      9.3      16.4
Cp       0.74     0.71
 

[marshmat]

Updated linesplan to reflect the last few days of reworking.... trying to unfold the developable hull into something that looked easy enough to build to justify the performance compromises, simply didn't look right. This is essentially the same shape with a little less rocker and soft bilges. Quarter-beam buttock angle is now 2 to 2.5 degrees. Cp for the main hull is 0.75 (!), I know that sounds high- but for reaching 20 knots in displacement mode, ie. Fn = 1.13, that's how things go....

[Ad Hoc]

Matt
wow, that was quick!
Just a few comments, if I may.
The rounding of the chine, may be i was not clear in my explanation, sorry. If, in the original, you had a small bilge radius instead of a chine, say some 100~200mm rad, from stn 7~10. Not a large one for the entire length as now drawn. Since some of the compromise is the ease of build. We have found on higher Fn boats a small rad in the chine, for 1/3 of the length aft reduces drag considerably.

The waterline, in plan view, the second outer one, from CL, this is near circular shaped, as it returns to the hull CL. These WLs would be better if the return is "straight" (ie perpendicular to the CL), as on the transom. This means as you go aft, the rise of keel, and the hull bottom, is a flat panel. If you had this "flat/straight" out to the small bilge rad, would make construction easier than the shaped you have. But i don't know your skills and ability. Only you can comment on that.

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Matt
Attached is an aft view of a catamaran. The WLs shown are straight as one goes aft. This enables the fitting of a water and duct so much easier, as well as 'encouraging' smooth flow into the duct. This is what I mean for yours too.

It is a 50m catamaran, round bilge, just for info.

[Willallison]

AH - what you describe is what I would refer to as a 'traditional' semi-displacement cat hullform. I take it that you retain the hard chine forward in an attempt to achieve water separation and hence lower wetted surface....
Clearly the flatter aft sections will go some way to increasing lift and thus reducing squatting further.
I'd be interested to know your experience of this shape as opposed to what is often referred to as the displaning hullform - where generally parabolic sections end in a canoe stern underneath and just short of a flat transom (as in the attached pic)
ps. Sorry Matt - won't nick your thread for long!

[Ad Hoc]

Willallison

The lines i posted are what 'we' describe as a displacement hull, not a semi-displacement hull. These hulls are not trying to plane, nor are they seeking any "lift". Any lift is a 'nice feature' if obtained (from endless sea trails almost no lift is observed). They simply "plough through" the water at high Fn's.

The 50m cat posted, her Fn is around 1, and hp/tonne roughly 40, with an L/D ratio around 9, and L/B of 16~18

A "typical" semi-displacement hull form falls outside such parameters. Their L/D ratios are generally 5~6, their L/B ratios around 5~8. So whilst the Fn's may be similar, their hull forms are very different.

On our Cat's, we generally don't have hard chines at all, the larger boats, these are all round bilge full on displacement type hulls. But on the smaller cats we do, for resistance, ie these are usually higher Fns. and are more "semi-planning", with their associated lower form factors, and consequently, are easier to build, but not the sole reason for the chine.

Typical form factors for such would be:
L/D = 7
L/B = 10

These, have a more pronounced hump in their resistance too. Hulls with high L/D ratios have almost no hump and very little trim, because they are displacement hulls and not semi-D. Think plank on edge.

Does this answer your Q?

Sorry Matt, for slight Hi-jacking..

[Willallison]

Originally Posted by Ad Hoc

Does this answer your Q?

Partly.... given the size of the vessel and her operating speed, she is clearly, as you say, operating fully in a displacement condition. I take it therefore that the flat aft sections are more to do with intake flows than anything else.
But, onto the part of my post that you didn't address... do you have any experience with the type of hullform that I posted and how it compares to 'your own'?
Clearly, it is not one suited to typical commercial applications where waterjets are the norm, so you may not have had reason to analyse the shape before...

[Ad Hoc]

W.
"..therefore that the flat aft sections are more to do with intake flows than anything else.."
Correct. The flat sections aft help fitting waterjet ducts. But, in Matt's case, will ahve the benefit of being flat, ie easy to build, even though no waterjets fitted.

It is difficult to see the shape of the hull, from that small pic, especially the transom, also where is the waterline on the hull, high up or low down? But it doesn't 'appear' to be any different from what I've done or seen before.

[marshmat]

For a waterjet installation, it is quite obvious that the flat aft sections are a highly desirable shape. Jets seem to like a smooth, straight stream of water at the intake, nothing rotating or swirling.

I don't think ease of construction is going to change very much whether the bilges are slightly rounded or significantly rounded, not in such a small, slender hull. The only construction options on the table are developed plywood or strip plank wood/epoxy; I prefer the shapes that can be done in strip plank and I doubt it will add much if any build time/complexity compared to developable shapes.

When fully loaded, I'm trying to pack a fair bit more weight in this thing than an 8-metre tri would normally carry, so she is necessarily somewhat fuller-bodied than would be 'ideal' for a typical, lighter boat. Also, I keep reminding myself that 20 kt / Fn=1.13 is a sprint speed, and normal cruising will be somewhat below that- 10 to 15 kt (Fn approx 0.5-0.8) will be where most of her underway time is spent, with the ability to handle Force 5 seas and high chop being more important than the 20 kt sprint capability.

Will- I've seen that "displaning" shape around, I think it was a Malcom Tennant idea? I have a few old sketches and preliminary calcs for hulls similar to that, but haven't been able to find enough published literature to really understand it. My understanding of it is that it's basically a canoe stern from baseline to a few inches below LWL, and a flat, sometimes even concave or down-angled, lifting surface above that for the aft few feet. Not unlike the box garboard of a Sea Bright skiff, really. The prop's in a protected spot, and the flat part is supposed to generate a bit of lift at higher speeds to counteract the tendency to squat. I've heard it works fairly well if it's usually run above Fn = 1.0 or so. You'd certainly have a hard time putting a waterjet in it. Never actually seen one, though- as I've mentioned, multihulls in general are a rarity in these parts, except for Hobies and the occasional F27.

[Willallison]

Apologies for the delay - it took me some time to track down a pic that adequately shows the transom....
Waterline on this type of hull is typically 100-200mm above "transom flat" - ie the transom is immersed at rest. The slight prop tunnells seen on this particular boat are not common to them all.
The shape is, as Matt suggests, generally attributed to Malcom Tennant and has proved to be an effective choice for many shaft drive (and more recently, outboard) installations. Clearly it has advantages in terms of driveline protection and in allowing a horizontal shaft, but there's little data out there as to whether it is any better / worse than a 'traditional' displacement cat hull - hence the questions as to whether AH has had any experience with it....

MM - I would expect the flatter aft sections to actually be more difficult to build, as they would require the strip-planks to be twisted quite a bit as they run aft. Still should be doable though - have you any early indication on the required core thickness yet? That may influence things....

[marshmat]

Will - My original plan for the hull skin was 9 mm (3/8") wood, with 2 mm epoxy/glass outside and 1 mm inside. That was calculated using a ply core and may change slightly with the revision to strip plank below the spray chine. Pending model tests, I think the shape as it is now should meet my performance objectives, and it is certainly buildable.

The thing that gets me about that 'displaning' stern is that it doesn't seem like something that's easy to design- there are several aspects (curvature and fore/aft slope of the flat surface, for instance) that will be hard to analyze with CFD, and hard to get right the first time without a lot of model tests. From what I've heard, it took a number of mediocre boats before Tennant and the other designers who use it really figured out the right balance of parameters to get it to work as intended. The knowledge of how to get the shape just right doesn't seem to be readily available, for obvious competitive reasons....

[Ad Hoc]

W
if that waterline is above the transom flat....it is just adding parasitic drag!

[Willallison]

MM - Yes - I've seen a number of variations on the theme. Many of them rather more complex with no obvious benefit. This, of course is the problem with any 'out of the ordinary' hullform - real analysis can only really be done by building / testing.

With strips approx that thickness, I agree you shouldn't have too much trouble getting pretty much any shape you want.

AH - The term displaning was coined to describe the way the two different parts of the hull behave at speed. As I said, at rest, the transom is immersed (as I assume would be the transom on the hull that you posted...) At speed the 'canoe body' of course remains fully submerged, creates no discernable lift and operates entirely in displacement mode. At speed, the 'transom flat' that extends beyond the canoe body appears to operate in more of a planing state: it creates measurable lift and the water breaks cleanly away from its trailing edge. In doing so, it also prevents the hull(s) from squatting.

[Ad Hoc]

W.
The stern on that hull is no different to that found on a SWATH, for clear and obvious reasons. However, if the hull shape is orientated to be like a canoe, and with the corresponding high L/D ratio, there will be no squatting or very little to actually write home about!

If the L/D ratio is low, then flat portion will drag terribly, most noticeably creating eddies on the chines. The flat portion may give the appearance of "providing lift" (or even 'some' minor lift), but at a significant increase in drag, owing to its shape and angle of incidence to the oncoming flow.

Hulls generally can't do both, no matter what the literature says, without a quid pro quo elsewhere. It is all swings and round abouts